The greenhouse effect is mainly caused by the accumulation of excessive carbon dioxide in the earth's atmosphere. Carbon dioxide, together with water vapor, methane and other so-called greenhouse gases, absorbs infrared radiation from the sunlight and at the same time blocks the heat from escaping to space. Some heat trapped in the atmosphere is transferred to the oceans and raises their temperature as well. Global warming eventually occurs. The increase in carbon dioxide in the atmosphere is mainly due to the use of fossil fuels such as coal, oil and natural gas. The plowing of soil and deforestation also indirectly increases the content of carbon dioxide in the atmosphere.
Photosynthesis is a natural process in which living systems remove carbon dioxide from the atmosphere and transform it into organic, carbon-containing compounds. The principal photosynthetic organisms in the carbon cycle are plants, phytoplankton, marine algae and cyanobacteria. They not only play an important role in converting light energy into chemical energy in order to serve as food for higher eukaryotes in the food chain, but are important in maintaining the level of carbon dioxide in the atmosphere by consuming carbon dioxide through photosynthesis. About 100 billion metric tons of carbon per year is bound into carbon compounds by photosynthesis.
Sunlight is an essential element for the light-dependent reactions in photosynthesis. Physically, sunlight can be resolved into a vast continuous spectrum of radiation called the electromagnetic spectrum. Radiation of each particular wavelength has a characteristic amount of energy associated with it. The light spectrum which demonstrates the relative effectiveness of different wavelengths of light for a specific light-requiring process is called an action spectrum. The light-dependent reactions are mainly carried out in the range between about 380 nm and about 750 nm in the electromagnetic spectrum, that is, the visible light portion of the electromagnetic spectrum. The electromagnetic waves outside this range such as ultra-violet (UV) and infra-red (IR) do not benefit photosynthesis and are even harmful for photosynthetic organisms.
In order for light energy to be converted into chemical energy in photosynthetic organisms, it must first be absorbed by a substance called pigment. However, not all wavelengths of light can be absorbed. Most pigments in photosynthetic organisms only absorb certain wavelengths of light which are suitable for carrying out photosynthesis while other wavelengths of light will be reflected or transmitted. The light absorption pattern of a pigment is known as the absorption spectrum of that substance. When the light absorption spectrum of a pigment and the action spectrum for a specific light-requiring process are similar in pattern, such pigment is regarded as effective for this specific light-requiring process.
For example, the light absorption spectrum of chlorophyll and the action spectrum for photosynthesis are similar and therefore chlorophyll is regarded as the principal pigment for photosynthesis. In particular, chlorophyll a is essential for the oxygen-generating photosynthesis by all photosynthetic eukaryotes and in cyanobacteria; other chlorophyll subtypes such as chlorophyll b (an accessory pigment in green algae, euglenoid algae and most plants), chlorophyll c (an alternative of chlorophyll b in brown algae and diatoms), bacteriochlorophyll (in some bacteria such as purple bacteria) and chlorobium chlorophyll (in green sulfur bacteria) are chemical variants of the basic structure of chlorophyll a with slightly different absorption spectrum. Two other classes of pigments involved in capturing light energy are carotenoids and phycobilins, where the former is mainly responsible for preventing photooxidative damage to chlorophyll molecules and the later is mainly found in cyanobacteria and red algae.
In order to fully utilize the whole spectra of sunlight, upconverting luminescent (UCL) and downconverting luminescent (DCL) materials have been used in recent years to convert the non-visible light into visible light suitable for photosynthetic organisms so that they can carry out the maximum light-dependent reaction. By using these luminescent materials, photosynthetic organisms can absorb a maximum amount of light energy at a suitable wavelength.
In U.S. Pat. No. 6,883,271, a device that converts UV light into growth-enhancing light for growth of plants or vegetables is disclosed. However, such a device is limited to the conversion of UV light and is unable to convert a wide range of non-visible light into a specific wavelength of visible light for specific photosynthetic organism. It is not a self-sustained system for growing photosynthetic organisms such as algae because algae rearing require water and nutrients circulating system as well as temperature control system. In U.S. Pat. No. 7,008,559, although the use of UCL and DCL materials as light converting materials in a greenhouse setting is disclosed, its design cannot transmit the visible light efficiently from different angles to each level of a multistory building; further it is limited to the growth of herbaceous and woody plants.
In addition to removing excess carbon dioxide from the atmosphere, the above-mentioned photosynthetic organisms are candidates for alternative energy because their by-product and/or biomass can be converted into biofuel. For example, oils derived from triacylglycerols in oil seed plants (e.g. soybean, sunflower and oil palm etc.) (Durrett et al., 2008) or microalgae (Hu et al., 2008) can be made into biodiesel. Algae is more preferable as a source of biofuel since a recent study reveals that algae have inherent advantages over other sources of biofuel such as higher yield, more rapid cell division and better quality (Robert, 2009).
Therefore, there is a need in the art for an improved system for treating unwanted carbon dioxide and waste heat and efficient use of light in the cultivation of photosynthetic organisms.